Ion implantation involves the introduction of impurities into a solid workpiece by directing an energetic beam of particles at the workpiece surface. When sufficiently energetic ions are used, the ions penetrate the workpiece surface and impregnate the near-surface region of the solid. Generally, the advantages of "doping" solids using ion implantation, over prior art methods such as high temperature diffusion or impregnation during crystalline growth, are (1) precise control over the type and amount (or "dosage") of impurity introduced and (2) precise control over the depth and uniformity of the impurity distribution.
While a number of scientific and engineering applications of ion implantation are known, a fairly recent and major application relates to the area of semiconductor device manufacture, where ion implantation has become a relatively standard doping technique in the fabrication of diodes, MOS transistors, resistors and the like. Typically, silicon is doped with boron, phosphorus or arsenic dopant ions having an energy of between about 3 and about 500 keV, yielding an implantation depth of about 100 to 10,000 Angstroms. Such a process thus places the implanted ions well below the surface, the depth of implantation being roughly proportional to the ionic energy. Upon annealing (heating to temperatures on the order of about 600 to 1000 degrees Centigrade), dopant concentrations of about 10.sup.14 to 10.sup.21 atoms per cubic centimeter are obtained.
A typical ion implantation apparatus includes at one end a gaseous source of appropriate material such as BF.sub.3 or AsH.sub.3, a means for controlling the flow of gas to an ion source, e.g. an adjustable valve, and a high voltage power supply which energizes the ion source so as to create a plasma therein, at pressures generally of about 10.sup.-3 torr. As the plasma is formed, the ions contained in the plasma are continuously extracted and accelerated through a voltage of about 10,000 to about 50,000 V. An analyzer magnet selects the ionic species of interest, which as a beam is then passed through a vacuum to the target. Examples of ion implantation devices known in the art include those disclosed in U.S. Pat. Nos. 4,008,683 to Rose; 4,346,301 to Robinson et al.; 4,498,833 to Hertel; and 4,628,209 to Wittkower.
U.S. Pat. No. 4,628,209 to Wittkower, for example, describes an ion implantation apparatus for doping a plurality of semiconductor wafers, wherein the wafers are supported on a rapidly rotating wheel and implanted by an ion beam which scans repeatedly over the wafer surfaces. The Wittkower invention includes an on-line method of monitoring the beam profile and thus improves one's control over the uniformity and dose of implantation.
The ion implantation devices of the prior art, though, still present several problems. First, metal oxides--e.g., gate oxides--on the semiconductor surface may be undesirably degraded by charge build-up during implantation. Second, as ion implantation technology has advanced, there has been an increasing and as yet unsatisfied demand for lower charge and particle build up, as well as for more precise control over uniformity and dose of implant as well as for improved surface smoothness.
A further problem with conventional implantation systems is the limited throughput which is achievable with them. That is, large volumes are not generally attainable. Most systems use some form of wheel on which to mount the semiconductor wafers. Normally these wheels are positioned in a horizontal plane for loading and then raised to a vertical position for implantation. The size of these wheels and the associated equipment is therefore necessarily limited by the amount of typical floor-to-ceiling space commercially available, which is usually eight to ten feet. Known exceptions to this are a system which mounts the wafers on the side of a rotating cylinder, as described in U.S. Pat. No. 4,346,301; and a system which uses a horizontal wheel and impinges the wafers from below, as described in "Computer Automation of High Current Ion Implanters", by Woodard et al., Nuclear Instruments and Methods in Physices Research, B6 (1985), pp. 146-153. These systems all require, during at least some stage of the wafer implantation process, that the wafers be mechanically clipped or otherwise held on a wafer support plate. Such contact with the wafer produces atmospheric pollutants, some of which inevitably end up on the wafers as unwanted particles.
Particle build up also results from handling and manipulating the wafers prior to loading onto and unloading off of a wafer-support wheel in nonvacuum conditions. Typically, wafers are provided to a transfer station in a carrier cassette in a clean-room environment in which people and other equipment are located. There is thus substantial opportunity for surface particle transfer and contamination of the wafers.